In recent years, water-absorbent resins for absorbing body fluids such as urine, sweat, and blood are widely used as a constituent element of sanitary materials such as paper diapers, sanitary napkins, incontinence pads, wound protecting material, and wound healing material. Such water-absorbent resins are utilized not only as sanitary materials, but also applied to various uses to absorb and retain water and absorb moisture, for example, building material, water retentive material for soil, drip absorbing and freshness retentive materials for food, and waterproof material.
Known water absorbing resins include partially neutralized and cross-linked acrylic acid (Japanese Publication for Unexamined Patent Applications No. (Tokukaisho) 55-84304, 55-108407, and 55-133413, and U.S. Pat. No. 4,654,039), hydrolyzed starch-acrylonitrile graft polymer (Japanese Publication for Examined Patent Application No. (Tokukosho) 49-43995, neutralized starch-acrylic acid graft polymer (Japanese Publication for Unexamined Patent Application No. (Tokukaisho) 51-125468), saponified vinyl acetate-acrylate copolymer (Japanese Publication for Unexamined Patent Application No. (Tokukaisho) 52-14689), hydrolyzed acrylonitrile copolymer or acrylamide copolymer, or cross-linked acrylonitrile copolymer and acrylamide copolymer (Japanese Publication for Unexamined Patent Application No. (Tokukaisho) 53-15959), cross-linked carboxymethyl cellulose, and cross-linked cationic monomer (Japanese Publication for Unexamined Patent Applications No. (Tokukaisho) 58-154709 and 58-154710), cross-linked isobutylene-maleic anhydrid copolymer material, cross-linked copolymer of 2-acrylamido-2-methylpropane sulfonic acid and acrylic acid, cross-linked polyethyleneoxide, and cross-linked copolymer of methoxypolyethylene glycol and acrylic acid.
All of the water-absorbent resins are in the form of particles or powder having a particle diameter of around 0.01 mm to 5 mm. The absorption rate of the water-absorbent resin is generally determined by the particle diameter. There is a tendency that the absorption rate of each particle increases as the particle diameter becomes smaller ("Polymers" Vol. 36, page 614, Polymer Association, 1987).
However, in actual, as the particle diameter becomes smaller, the liquid permeability for allowing aqueous fluids, for example, body fluids, to flow between the particles, is lowered. Namely, a so-called gel blocking phenomenon occurs. Therefore, when using the water-absorbent resin, it is necessary to select an optimum particle diameter by considering the absorption rate and the liquid permeability. The tendency of causing a gel blocking phenomenon becomes higher as the absorption rate of the water-absorbent resin increases. The main causes of the gel blocking phenomenon are a decrease in the void space and an increase in tack between particles after being swelled, under pressure.
In order to improve the water absorption characteristics of the water-absorbent resin, particularly, the absorption rate, various production methods and modification methods of water-absorbent resins have been proposed as shown below. More specifically, as the production and modification methods of water-absorbent resins, for example, the following two methods have been proposed. 1 Application of secondary cross-linking treatment, i.e., improving the cross-link density in the vicinity of a particle surface. 2 Increasing the particle surface area by granulation, foaming, formation of pores, or the like.
The method 1 includes methods which use the following materials as a surface cross-linking agent. Namely, a method using polyhydric alcohol; a method using a polyglycidyl compound, a polyaziridine compound, a polyamine compound and a polyisocyanate compound; a method using glyoxal; a method using polyvalent metal salt; a method using a silane coupling agent; a method using a mono epoxy compound; a method using a polymer containing an epoxy group; a method using an epoxy compound and a hydroxy compound; and a method using alkylene carbonate.
For instance, the following methods were also proposed. A method in which a cross-linking reaction is performed under the presence of inactive inorganic powder (U.S. Pat. No. 4,587,308). A method in which a cross-linking reaction is performed under the presence of dihydric alcohol. A method in which a cross-linking reaction is performed under the presence of water and an ether compound. A method in which a cross-linking reaction is performed under the presence of alkylene oxide added monohydric alcohol, organic acid salt, or lactam. A method in which more than one kind of cross linking agents having different solubility parameters are used. Moreover, the methods for improving the cross-link density in the vicinity of the particle surface are disclosed in U.S. Pat. No. 4,666,983, U.S. Pat. No. 5,140,076 and U.S. Pat. No. 5,229,466, and Japanese Publication for Unexamined Patent Applications No. (Tokukaisho) 59-62665 and No. (Tokukaihei) 5-508425.
As the method 2, for example, a method using a blowing agent during polymerization or cross-linking was proposed. The method using a blowing agent includes, for example, methods in which a cross-linked structure is introduced into a linear water-soluble polymer while performing neutralization using a blowing agent such as carbonate (U.S. Pat. No. 4529,739 and U.S. Pat. No. 4,649,164), methods in which carbonate salt is added to monomers (Japanese Publication For Examined Patent Applications No. (Tokukosho) 62-34042, No. (Tokukohei) 2-60681, and No. (Tokukohei) 2-54362, and U.S. Pat. No. 5,118,719, U.S. Pat. No. 5,154,713 and U.S. Pat. No. 5,314,420), a method in which monomers are polymerized using a microwave under the presence of carbonate salt (U.S. Pat. No. 4,808,637), methods in which an organic solvent having a boiling point within a range of from 40.degree. C. to 150.degree. C. is added to a specified monomer and then polymerized (Japanese Publication For Unexamined Patent Application No. (Tokukaisho) 59-18712, and U.S. Pat. No. 4,552,938, U.S. Pat. No. 4,654,393 and U.S. Pat. No. 4,703,067), and methods in which a hydrophobic organic solvent is added and polymerized under specified pressure (U.S. Pat. No. 5,328,935 and U.S. Pat. No. 5,338,766). Additionally, methods in which a blowing agent is added after polymerizing monomers were also proposed (Japanese Publication For Unexamined Patent Applications No. (Tokukaisho) 56-13906, No. (Tokukaisho) 57-182331, and No. (Tokukaisho) 57-208236).
Furthermore, the following methods were also proposed. A method in which a polarity is given to particles using a microwave (WO No. 91/02552). Methods in which fine particles are made into secondary particles by granulation (WO No. 93/24153, U.S. Pat. No. 5,002,986, U.S. Pat. No. 5,300,565, U.S. Pat. No. 5,140,076 and U.S. Pat. No. 4,732,968).
With the use of the methods 1 and 2, it is possible to improve the absorption rate of the water-absorbent resin to some extent.
However, the water-absorbent resin prepared by cross-linking cannot achieve a high absorption rate which is required when it is used, for example, in sanitary materials. In addition, the water-absorbent resin prepared by cross-linking while foaming a linear polymer does not have sufficient absorbent capacity (water retention capacity), and requires a high cost. Whereas the porous water-absorbent resin prepared by foaming while polymerizing monomers is excellent in terms of the absorption rate and cost. However, it is difficult to control the timing of foaming, and cannot achieve a uniform pore diameter. Thus, these water-absorbent resins failed to sufficiently improve various characteristics related to the dispersion of aqueous fluid, water-soluble component content, residual monomer content, and dry touch (these characteristics will be explained later).
More specifically, the water-absorbent resins obtained by the above-mentioned production method or modification method have such disadvantage that the mutual balance of conflicting characteristics, such as the dispersion of aqueous fluid, the water-soluble component content, and dry touch, is not satisfactory. Namely, the above-mentioned conventional water-absorbent resins do not have sufficiently improved water absorption characteristics, and cannot provide high water absorption characteristics which are required when used, for example, in sanitary materials.
An object of the production method and modification method is to produce a water-absorbent resin capable of promptly absorb aqueous fluid when the water-absorbent resin comes into contact with the aqueous fluid. Therefore, these methods are designed without substantially considering the water absorption characteristics that are required of the water-absorbent resin when the water-absorbent resin is used in a sanitary material, particularly, when a large amount of water-absorbent resin is used in sanitary material to reduce the thickness of the sanitary material.
In the sanitary material using a large volume of the water-absorbent resin, it is necessary to improve the absorption. However, if the absorption rate is increased, the gel blocking phenomenon tends to occur. In order to reduce the incidence of the gel blocking phenomenon, for example, an attempt has been made to improve the elasticity of gel. However, if the elasticity of gel is improved, the water retention capacity of the water-absorbent resin is lowered. Therefore, even if the water-absorbent resin having improved absorption rate and elasticity of gel is used in the sanitary material, it is hard to say that the sanitary material is prevented from leakage. Thus, there is a demand for a water-absorbent resin capable of keeping various characteristics such as absorption rate and water retentive ability, and achieving improved dispersion of aqueous fluid between particles after absorption, that is a characteristic conflicting the above characteristics.
The present invention was carried out to solve the above conventional problems. An objective of the present invention is to provide a water-absorbent resin having excellent water absorption characteristics such as the dispersion and absorption rate of aqueous fluid, water retentive ability and dry touch, lower water-soluble component content, and lower residual monomer content. Another objects of the present invention to provide a process for producing the water-absorbent resins, and a water-absorbent resin composition using the water-absorbent resin.